Wheeled vehicles depend upon the maintenance of an adequate frictional contact between the driven wheels of the vehicle and the ground. When the driving force at the driven wheels exceeds a value which is related to the weight acting on the driven wheels and the co-efficient of friction of the ground surface, the driven wheels slip with respect to the ground, thus exhibiting the phenomenon known as wheel spin. During normal operation of a vehicle on surfaces of relatively high co-efficient of friction, such as dry tarmac, wheel spin is not a problem. However, on surfaces of relatively poor co-efficient of friction, such as water logged tarmac, ice, and muddy surfaces, poor driving technique can give rise to wheel spin. When a wheel spins excessively, the loss of proper frictional contact with the ground allows the wheel to slip sideways and, in extreme cases, can result in spinning or excessive yawing of the vehicle i.e. lateral instability. Small amounts of wheel spin do not result in substantial lateral instability and may even be beneficial for maximizing acceleration of the vehicle or preventing damage to the vehicle transmission caused by careless or over-enthusiastic driving techniques. However, excessive amounts of wheel spin should be avoided and, for this purpose, so-called "traction control" systems have been developed.
A known type of traction control system reduces the output demand of an internal combustion engine as a predetermined function of the amount of wheel spin above a predetermined threshold value. For instance, a proportional/integral/differential control function of wheel spin is used to reduce the engine output demand. However, in some circumstances, such a control function does not allow the energy stored in the vehicle engine to be reduced sufficiently quickly, so that loss of vehicle stability can occur.